The Eph subfamily represents the largest branch of membrane-spanning tyrosine kinases known to date. Because it has been identified only recently, it has not yet been extensively characterized. The broad long term objectives of this proposal are to elucidate the functions of the Eph subfamily of tyrosine kinases during retinal morphogenesis and in the mature retina. These studies will suggest possible implications of the Eph-related kinases in the etiology of pathological conditions affecting the retina. The retina also represents a convenient model system to elucidate the roles of the Eph-related kinases in the central nervous system. Different Eph-related kinases have distinct tissue distributions in the adult and, thus, presumably have distinct functions. At least six of the Eph-related kinases that we have identified, namely Cek4, Cek5, Cek6, Cek8, Cek9 and Cek10, are expressed in the developing and adult retina suggesting an important role for the Eph subfamily in the retina. The catalytic activity of the Eph-related kinases is likely to be responsible, at least in part, for the elevated protein tyrosine phosphorylation that is observed both in the developing and mature retina. Cek5, for example, is highly expressed and catalytically active during the differentiation of the retina. This suggests that Cek5 plays an active role during retinal morphogenesis and that mutations of the Cek5 gene are likely to disrupt the normal development of the retina. In addition, the gradient-like distribution of Cek5 in the retina and optic nerve indicates that Cek5 may be involved in the determination of the positional identity and synaptic specificity of retinal ganglion cells. Similarly, the elevated expression of Cek8 throughout the developing optic nerve suggests a role in axonal growth and guidance. To gather information about their possible functions, the spectrum of Eph-related kinases that are expressed in the retina will be identified and their expression levels, spatial distribution and extent of activation in the developing and mature retina will be characterized. The expression and activity of specific Cek Eph- related kinases will be perturbed with antibodies, dominant negative forms, anti sense sequences and ligands. These studies will be performed in vivo and in vitro to address directly the functions of the Eph subfamily in the retina. The identification of the cellular processes that are regulated by the Eph-related kinases, which may include cell fate determination, cell proliferation and differentiation, nerve cell survival and regeneration, cell adhesion and axonal guidance, will allow the manipulation of such processes with possible therapeutic outcomes.